Simply put - the more power you put on the loop, the higher the transponder will read. Note that with ActivePro V1 / V2 transponders this can also result in reduced accuracy.
Depending on the type of sport, choose an appropriate value for Loop Power. For example for cycling (high precision needed, transponder fixed on the bike, thus close to the loop) choose a low value like 20%. However, if participants wear the transponder on the wrist and can finish with their arms raised, choose a high loop power like 80% or 100%.
Depending on what materials are underground at the timing point, the maximal read height with 100% loop power is 2-2.5 meters. Metal in the ground will reduce the read height.
You can see some guidelines for loop width / loop power and the resulting max detection height here